Samacheer Class 7 Science - Changes Around Us

The given examples can be classified into physical and chemical changes. Physical changes are those in which no new substance is formed and the change is generally reversible. Examples of physical changes include: bending a paper clip, pounding silver into thin plate, rolling chapathi dough into thin wire, occurrence of day and night, blinking of eyelids, occurrence of a thunderstorm, rotation of the earth, formation of eclipses, painting the grill, and a rough piece of wood being sanded and polished resulting in change in texture. In these changes, only the physical properties like shape, size, texture, or appearance change, but the substance remains the same. Chemical changes are those in which new substances are formed and the change is generally irreversible. Examples of chemical changes include: rusting of an iron nail, eruption of a volcano, burning of a matchstick, and dosa being made from batter. In these changes, the chemical composition of the substance changes, resulting in the formation of entirely new substances with different properties from the original materials.

Physical changes and chemical changes differ in several important ways. In physical changes, no new substance is formed; only the physical properties like size, shape, color, or state of matter change. Physical changes are generally reversible, meaning the original substance can be recovered. Examples include melting of ice, tearing of paper, freezing, and evaporation. In contrast, chemical changes result in the formation of new substances with different properties from the original reactants. Chemical changes are generally irreversible, meaning the original substance cannot be easily recovered. The chemical composition of the substance changes during a chemical change. Examples include burning of paper, photosynthesis, digestion of food, and rusting of iron. In physical changes, the identity of the substance remains the same, whereas in chemical changes, the identity of the substance changes completely.

A change can occur in a substance through alterations in its properties such as color, texture, and state. These changes happen when the substance is subjected to external conditions or forces. A change in a substance can be either physical or chemical in nature. In physical changes, the properties like size, shape, color, or state of the substance change, but no new substance is formed. In chemical changes, the chemical composition of the substance is altered, resulting in the formation of new substances with entirely different properties. Changes can be brought about by applying heat, light, pressure, or through chemical reactions. The type of change that occurs depends on the nature of the alteration and whether new substances are formed during the process.

Dissolution of sugar is a physical and reversible change. When sugar is dissolved in water, it disappears from view, but if we taste the solution, the sugar is still present in dissolved form. The sugar molecules remain unchanged; only their physical state changes as they mix with water molecules. If water is evaporated from the solution, we can recover the sugar crystals in their original form, proving that the change is reversible. No new substances are formed during this process. In contrast, burning of sugar is a chemical change. When sugar is heated in a flame, it undergoes a chemical reaction with oxygen present in the air. The chemical bonds in the sugar molecules break, and new substances such as carbon dioxide and water vapor are formed. Energy is released in the form of heat and light, and black carbon residue may remain. This change is irreversible because the original sugar cannot be recovered, and entirely new substances with different properties have been created. The key difference is that dissolution is only a physical change of state, while burning involves a permanent alteration of the chemical composition of the substance.

Digestion is a chemical change because it involves the breaking down of food into entirely new substances with different chemical compositions. When we eat, food undergoes both mechanical and chemical digestion. Mechanical digestion occurs in the mouth, stomach, and small intestine, where food is physically broken into smaller pieces. However, the real transformation happens through chemical digestion, where enzymes and digestive acids chemically alter the structure of food molecules. Complex substances like carbohydrates, proteins, and fats are broken down into simpler molecules such as glucose, amino acids, and fatty acids. These new substances have completely different properties and chemical compositions from the original food. Moreover, digestion is an irreversible change because we can never recover the original raw food material in its initial form from the digested products. Once food has been chemically broken down and absorbed by the body, it cannot be reassembled into the original food. This permanent and irreversible transformation of food into new substances with different chemical properties is the defining characteristic that makes digestion a chemical change.

First the ring in the iron blade of spade is heated.
Heating of the blade leads to its expansion and thus the ring gets bigger.
This happens because metals always expand on heating.
The metal blade is then fitted easily into the wooden handle.
After this, cold water is usually poured on the iron blade which leads to the contraction of the expanded iron blade.
The spade can also be left to cool down, undisturbed so that it is firmly fixed, (vu) The blade is attached firmly to the wooden handle of a spade in this process.
X.Higher order Thinking questions :

When a very hot glass is suddenly placed in cold water, it cracks due to uneven cooling. This change indicates the property of thermal expansion and contraction of glass. Glass expands when heated and contracts when cooled. When the hot glass comes into contact with cold water, the outer surface of the glass cools down rapidly and contracts quickly. However, the inner portion of the glass remains hot and takes longer to cool down, so it contracts more slowly. This uneven contraction creates internal stress and tension within the glass. The outer surface tries to contract more than the inner portion can accommodate, causing the glass to crack or shatter. This is a physical change because the chemical composition of the glass remains the same; only its physical shape and structure are altered. However, it is an irreversible change because once the glass cracks, it cannot be restored to its original unbroken form simply by reversing the temperature change. This demonstrates how rapid temperature changes can cause permanent physical damage to materials like glass.

Boiling of water is a physical change because only the physical state of water changes, not its chemical composition. When water is boiled, it is converted from liquid state to gaseous state in the form of steam. The water molecules remain H₂O molecules; no new substances are formed. If we cover the beaker with a lid, the steam condenses back into liquid water when it cools down. This reversibility proves that boiling of water is a physical change since the original substance can be recovered unchanged. In contrast, boiling of an egg is a chemical change because the chemical nature of the egg changes permanently. When an egg is boiled, the proteins in the egg white and yolk undergo a chemical transformation called denaturation. The heat breaks the chemical bonds in the protein molecules, causing them to unwind and form new structures. A boiled egg has completely different properties from a raw egg: the texture becomes solid and firm, the color changes from transparent to opaque white and yellow, and the taste and smell are different. These new properties indicate that new substances with different chemical compositions have been formed. The change is irreversible because we cannot convert a boiled egg back into a raw egg by any simple means. Therefore, boiling of water is physical, while boiling of an egg is chemical.

Wet clothes dry faster when we unfurl or spread them out because the rate of evaporation depends directly on the surface area exposed to air. When we spread clothes wide and hang them separately, we increase the total surface area from which water can evaporate. The greater the surface area in contact with air, the more water molecules from the wet cloth can escape into the atmosphere as water vapor, and therefore the faster the evaporation occurs. In contrast, if we simply drape wet clothes over a clothesline without unfolding them, the clothes remain bunched up with a smaller exposed surface area, which results in slower evaporation and longer drying time. By unfolding and spreading the clothes, we maximize the exposure of the wet fabric to air and sunlight, allowing water to evaporate more rapidly from all sides of the cloth. This principle of increased surface area leading to increased evaporation rate is the reason why we unfurl clothes rather than keeping them folded or bunched together when drying them.

Magnesium ribbon must be cleaned by rubbing its surface with sandpaper before burning because magnesium reacts with oxygen in the air to form a layer of magnesium oxide on its surface. This oxide layer prevents the magnesium underneath from reacting properly with oxygen during the burning process. By removing this oxide coating with sandpaper, we expose the clean magnesium metal surface, which allows the desired chemical reaction to occur. When clean magnesium ribbon is held over a flame with tongs, it burns in air with a bright, dazzling white flame. During this combustion, magnesium reacts with oxygen to form magnesium oxide, which appears as a white ash. The chemical reaction can be represented as magnesium combining with oxygen to produce magnesium oxide. This is a highly exothermic chemical change that releases a large amount of heat and light energy, which is why the flame is so bright and intense. The white ash left behind is the product of this chemical reaction.

Physical changes are those in which only the physical properties of substances change without altering their chemical composition. The physical changes in the list are: chopping wood, where the size and shape of wood pieces change but the wood remains wood; mixing sand and water, where two substances are physically combined but retain their individual properties; crushing a can, where the shape of the can is altered but the metal composition remains unchanged; and mixtures of different colored buttons, where buttons of different colors are simply combined without any chemical reaction. Chemical changes are those in which new substances with different chemical properties are formed. The chemical changes in the list are: rusting of iron, where iron reacts with oxygen to form iron oxide; digestion of food, where complex food molecules are broken down into simpler substances; boiling egg, where proteins are chemically altered by heat; rotting banana, where organic compounds decompose into new substances; and burning of wood, where wood reacts with oxygen to produce carbon dioxide, water vapor, and ash.

Yes, photosynthesis is definitely a chemical change. Photosynthesis is the process through which plants convert light energy into chemical energy stored in food molecules. In this process, water and carbon dioxide combine together in the presence of sunlight to form glucose and oxygen. The chemical equation for photosynthesis shows that the reactants (water and carbon dioxide) are completely transformed into new chemical substances (glucose and oxygen) that have entirely different properties from the original materials. Since new chemical species with different chemical compositions and properties are formed, and the process involves breaking and forming of chemical bonds, photosynthesis is clearly a chemical change. This is different from physical changes where only the form or appearance of a substance changes without altering its chemical composition. The formation of glucose, which is a complex organic compound, from simple inorganic substances demonstrates that a chemical change has occurred at the molecular level.